From users'point of view, there are currently two types of computer displays for 3D (three-dimensional) visualization: (1) Perspective presentation: presenting perspective views of 3D objects on a conventional 2D display (sometimes called "2.5D"); and (2) 2D display with depth: presenting images with a sense of depth on physically-2D display, such as holographic or stereoscopic displays. Perspective presentation of 3D images can be created by 3D software tools on a 2D screen. However, there are still many cases where 2.5-D images can not convey the information clearly without confusion. For example, many users of 3D CAD (computer aided design) tools have a hard time in positioning objects in a 2.5-D perspective graphics environment. Holographic and stereoscopic displays have been developed to provide 3D illusion. One major issue of computer generated holography is the vast computation power required for calculating the hologram corresponding to the 3D image, which makes the technology still far from practical use.Benton 1992 ,Report 1990! The main problem with stereoscopic displays is the need of helmets and glasses. Special screen illumination and filtering technologies have been developed to remove the need for glasses in stereoscopic type display. But display resolution is generally sacrificed and head tracking is necessary, which means only one user can see 3D) images at any time. Walko 1995, Shandle 1993!
This invention relates generally to new and improved approaches for displaying volumetric three-dimensional (3D) images. A volumetric 3D display is capable of displaying 3D volumetric images in a spatial region. Each "voxel" (relative to "pixel" on a 2D display) on a 3D image displayed by a volumetric display locates actually and physically at the spatial position where it is supposed to be, and light rays travel directly from that position toward omni-directions to form a real image in the eyes of viewers. As a result, a volumetric 3D display has 360.degree. view angle, allows multiple viewers, does not require glasses, and can be a very useful new display device. Potential applications in medical displays, radar/sonar displays, computer aided design, and electronic games are expected.
There have been several previous approaches on displaying volumetric 3D images. One of the early approaches is the Varifocal Mirror. It consists of a vibrating mirror and a stationary CRT (cathode ray tube). A series of cross sectional images are displayed in sequence on the CRT, which, as viewed through reflection from the vibrating mirror, form a volumetric 3D image. Traub 1967! This approach has a very limited view angle because images are inside the mirror.
A second type of volumetric display uses a rotating or reciprocating LED (light emitting diode) matrix. A sequence of 2D cross sectional images is displayed as the LED matrix rotates or moves in space to create 3D images. Berlin! However, moving large area LED panels at high frequency creates reliability problem and signal coupling issues.
Another type of volumetric 3D display uses the intersecting light spot of a scanning laser beam on a moving screen or disc, which sweeps across a volume, to generate 3D images. Williams 1989, Soltan 1992, Batchko 1992! This approach was based on well established laser beam scanning technology. However, this "point scanning" method seriously limits data rate, resolution, brightness, and color. Laser scanning rate is limited by the number of reflecting faces of the polygon mirror and the resonant frequency of scanner structure. It has been estimated that the number of data point per volume is limited to about 30,000, which is far from enough for high quality 3D images. Solomon 1993! As a result, only wire frame or coarse bits images are possible.
Still another approach uses a moving screen coated with phosphor to receive electron beams emitted from stationary sources.Blundell 1994! This is also "point scanning approach" and has similar problems as the scanning laser approach. There is another type of approaches which uses the intersection of two laser beam or two electron beams to excite an opto- or electro-sensitive material in a box or sphere.Rowe 1977, Korevaar 1989! This "two step excitation" method again suffers the "point scanning" limitation.
There have been recent works using whole frame display instead of "point scanning". One work by NIST (National Institute of Standards and Technology) uses a piezo-based fast focusing lens to project image frames to a stack of PDLC screens Paek 1996!. Another work uses a fixed focal-length projection lens to project images from a stack of LCD panels Hattori 1992!. Both approaches have limited resolution because the number of LCD panels or screens in the stack in physically limited.
A practical volumetric image display must have adequate resolution, enough brightness, and allow viewing by multiple users. None of the previous techniques reviewed above can satisfy the three requirements, due to the limitation of their structure, or resolution, or the "point scanning" nature.